Conventionally, a loudspeaker system in which a gas adsorber is provided in a cabinet has been proposed (e.g., a loudspeaker system disclosed in Patent Document 1). FIG. 19 is a partial cross-sectional view showing a main portion of a loudspeaker system 91 disclosed in Patent Document 1. The loudspeaker system 91 includes a cabinet 910, a loudspeaker unit 911, a gas adsorber 912, and a bag 913. The cabinet 910 includes a plate-shaped front wall 9101 and a curved side wall 9102. The loudspeaker unit 911 is an electrodynamic loudspeaker. The loudspeaker unit 911 is attached to the front wall 9101. The gas adsorber 912 includes a porous material having a large number of pores, such as activated carbon. The pore has a size of, for example, the order of nanometers, and therefore, can physically adsorb air. The gas adsorber 912 is enclosed in the bag 913 which is shaped in a tube. The bag 913 is provided in a curved portion of the side wall 9102 in the cabinet 910.
An operation of the loudspeaker system 91 thus configured will be described. When an electrical signal is applied to the loudspeaker unit 911, the vibration of the loudspeaker unit 911 changes air pressure in the cabinet 910. Due to the change in the air pressure, air passes through the mesh of the bag 913 and is then physically adsorbed by the gas adsorber 912 enclosed in the bag 913. As a result, the change in the air pressure in the cabinet 910 is suppressed, so that the volume of the inside of the cabinet 910 is equivalently increased. As a result, a small cabinet can reproduce low frequency sound as if the same loudspeaker unit were attached to a large cabinet.
Here, the loudspeaker system employing the gas adsorber has a problem that as humidity increases, water vapor is adsorbed by the gas adsorber, resulting in a reduction in the volume increasing effect.
Therefore, in the conventional the loudspeaker system 91, the reduction in the volume increasing effect is prevented by applying a hydrophobic treatment to the gas adsorber 912 and the bag 913.
Also, a microphone device employing a dehumidifier has been proposed as disclosed in Patent Document 2. FIG. 20 is a cross-sectional view showing a structure of a microphone device 92 disclosed in Patent Document 2. In FIG. 20, the microphone device 92 includes a container 920, a diaphragm 921, a back electrode 922, an insulating support 923, an amplifier 924, a dehumidifier 925, an insulating support 926, and a connector 927. The container 920 has openings 921h to 923h. The dehumidifier 925 includes an active device in which a porous electrode is formed on each surface of a solid electrolyte film having hydrogen ion conductivity. A DC power supply is connected to the dehumidifier 925, though it is not shown in FIG. 20.
An operation of the microphone device 92 thus configured will be described. The diaphragm 921 vibrates in accordance with an audio or music signal transferred via the opening 921h from the outside. The capacitance between the diaphragm 921 and the back electrode 922 varies due to the vibration of the diaphragm 921, and an electrical signal indicating the variation is output to the amplifier 924. The electrical signal is amplified by the amplifier 924 before being output through the connector 927. Note that the container 920 has the opening 923h so as to adjust a change in air pressure in the container 920 which is caused by a change in outside air pressure.
Here, in the conventional microphone device 92, outside damp air enters through the opening 923h for adjusting the air pressure. If the damp air reaches a space between the diaphragm 921 and the back electrode 922, noise occurs, which is a problem.
Therefore, in the conventional microphone device 92, a DC voltage is applied between the electrodes of the dehumidifier 925, which in turn performs ion decomposition, thereby discharging damp air through the opening 922h to the outside. Thus, in the conventional microphone device 92, the occurrence of noise is prevented by discharging damp air to the outside using the dehumidifier 925.    Patent Document 1: Japanese National Phase PCT Laid-Open Publication No. 2004-537938    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-343318